During the last century, civil wars have been getting longer. Between 1900 and 1944, they tended to last just one-and-a-half years. By 1999, they stretched to an average of 15.

Will Syria, like Libya's eight-month revolution, defy this trend and wrap things up within two years? Or, like Lebanon next door, is it fated to a catastrophic, slow-motion implosion?

The answer remains unclear, principally because the end of the regime's grip on Damascus is not the end of the story. We might see a messy retreat of loyalist forces out of the capital and toward the Levantine highlands and coastal plains. Or the civil war might mutate into a fratricidal battle, pitting the anti-Assad jihadist factions against moderate rebels, or Kurds against Sunnis, or militia against militia.

Now, the good news: these disturbing possibilities notwithstanding, we are witnessing the beginning of the end for the Assad dynasty, the last republican monarchy of the Middle East. And events, as they often do, are moving quicker than our policies.

At the end of November, the CIA is reported to have estimated that President Bashar al-Assad had just eight to 10 weeks left. With our attention on Gaza two weeks ago, we missed the turning of the tide.

Syrian rebels of all stripes began over-running military bases. They seized heavy weaponry and acquired sophisticated anti-aircraft weapons, which they immediately put to use.

As the regime haemorrhaged airbases, not only did this sap the government's key advantage – airpower – but it also set in motion a virtuous circle: the spoils of war taken from one base made it easier to capture the next one. What all this means is that Syrian rebels are no longer just harassing checkpoints or sniping at convoys.

Then, after months of indecisive fighting around Damascus, the capital came under intense attack. The airport was rendered unusable, EU and UN diplomats left the country and the regime compounded its isolation by shutting down the country's internet.

According to the 'New York Times', Russian envoys to Assad "described a man who has lost all hope of victory or escape".

And he is not the only one. This week, the regime's most senior Christian figure, foreign-ministry spokesman Jihad Makdissi, defected. Meanwhile, the deputy foreign minister visited Cuba, Venezuela and Ecuador – presumably to search out opportunities for asylum.

Amidst these developments, the West has struggled to respond. The US, emerging from its election-season paralysis, is agonising over how far it should support the opposition, let alone intervene.

It was spooked last year when it realised the blurriness of the division between moderate and extreme rebel groups on the ground. Britain and France are mulling over the provision of arms on the basis that the risks are outweighed by the importance of shoring up moderate rebels. Turkey is deeply frustrated, having failed to secure a no-fly zone. By the time everyone makes a decision, the whole thing could well be over.

The unprecedented rebel military advances mean that we must start thinking about the endgame. We should avoid the illusion of control. We have only limited influence over the direction in which Syria goes, but there are constructive steps that we can take.

Although Assad would gain little from using chemical weapons, desperate regimes make strange choices. Last year, Colonel Gaddafi pointlessly fired ballistic missiles toward rebel-held territory a week before his regime collapsed. In 1991, Saddam Hussein lobbed 42 missiles at Israel. Nato's deployment of the Patriot missile-defence system to Turkey is therefore prudent.

However, the greater danger is that chemical weapons may be seized by extremist groups.

Unless we can be sure that Syrian army units guarding chemical weapons will retain absolute control, it may become necessary to secure, remove or destroy at least some of the stockpiles. This would require a US-led Jordanian force, assisted by trusted Syrian rebels, with Britain and other states likely to play a role.

In the interim, the West should be reasserting the offer of safe passage for Assad. However improbable, it would be far preferable to a last stand that leaves Damascus in ruins. We should also be thinking of ways to protect Syria's minorities from what could be an horrific retribution.

Regrettably, this can probably only be done by keeping the Syrian armed forces from dissolving, as occurred in Iraq in 2003. We should also be unafraid of talking to Russia and Iran about these contingencies.